The present invention generally relates to a method and an apparatus for measuring a required feature of a layer of known material of an object to be polished during a polishing process. More particularly the present invention relates to an apparatus and a method of in-situ measurement of layer thickness for chemical mechanical polishing (CMP).
Integrated circuits are typically formed on substrates, particularly silicon wafers. The integrated circuits are formed by depositing different layers of conducting, semiconducting or insulating nature. After deposition of each layer, features of the electrical circuits are incorporated, e.g. by etching. During the sequential procedure, the upper surface of the substrate becomes more and more non-planar. Thus, the surface of the substrate has to be planarized in order to provide a substantially planar surface.
For example, such planarization can be achieved by chemical mechanical polishing (CMP). In general, during a CMP process a substrate is mounted to a carrier or polishing head. The exposed surface of the substrate is moved against a rotating polishing pad on a polishing platen. A polishing slurry is distributed over the polishing pad. The slurry includes an abrasive component and at least one chemically reactive agent; thus, an abrasive chemical solution is provided at the interface between the pad and the wafer in order to optimize the polishing.
In general, it is desirable to control CMP processes, in order to find an endpoint for polishing or to determine the thickness of a layer.
According to a prior art control process pre and/or post measurement of wafers with either manual or automatic feedback control is performed. Systems are available by which it is possible to measure wet wafers immediately before and after polishing. Due to the monitoring of the condition of the surface before and after polishing it is possible to change the polishing parameters, and therefore, to optimize the polishing during a series production. However, such pre and/or post measurement method bears the disadvantage that the first or the first few wafers have to be polished without optimized parametersxe2x80x94they are polished xe2x80x9cblindxe2x80x9d. Further, during a series production the appearance of the wafers might vary; the pre and/or post measurement method is not capable to consider such variations of the wafers.
In order to find the correct endpoint for polishing several endpointing methods are available. Current methods include measuring temperature, friction, vibration, sonic level, and frequency. Further, various optical measurements are available, e.g. reflection properties measurement methods. Unfortunately, these processes do not work for all substances, in particular when an oxide is polished. However, a major portion of CMP processing is polishing metal films. An optical metrology film measurement technique cannot be used for measurement of opaque films.
Acoustic wave techniques have been applied in the prior art to measure layer thicknesses in a measurement environment where no mechanical treatment is done in parallel, i.e. no in-situ measurement by acoustic wave techniques bas been demonstrated for CMP.
Accordingly, there is a need for a measuring method that can be employed to measure the thickness of opaque films or layers without mechanical contacting or damaging of the layer. A further object of the present invention is to provide a measuring apparatus for thickness measurements of a thin layer, especially of opaque layers. The method and the apparatus of the present invention should be applicable to CMP endpointing.